In the perfect conditions, installed by a professional using the highest quality mortar and grout during optimal drying conditions, tile installations can last generations. However, human error as well as natural disasters can cause damage. Hollow floor tiles are one of the casualties of water damage, and knowing how to fix the issue can make your life a little less stressful.
Nature of Mortar
When a tile installation is first installed, the thinset mortar is troweled onto the floor with a notched trowel. The depth of the trowel, or the depth of the notches that apply the thinset, vary based on how large the tile is. Larger tiles require larger notches, or more thinset, to bond properly with the back of the tile. If excess moisture is present during the curing phase, either in the form of excessive humidity or water damage from a flood, the mortar will not bond with the tile and leaves hollow-sounding tiles behind. Tile that has properly bonded with mortar that has been allowed to cure should rarely become hollow, as water does not generally affect cured concrete (as the underwater piers and docks of the Roman Empire, still whole and durable under the Mediterranean and other bodies of water, attest).
Individual Tiles vs. Whole Floor
While an individual tile is not necessarily cause for alarm, several tiles across an area, or, worse yet, random tiles across the entire installation surface, can be a reason to fret. Before you start pulling your hair out over an expensive remodel, check the floor to be sure. Tap the tiles lightly with your knuckles to check for hollow sounds, which resemble an echo of your rapping. If you are looking at a single piece here, another piece over there, and only portions of the tiles sound hollow, you have individual tiles and should not worry. Several tiles together, or worse yet, tiles across the whole floor, mean there is significant failure, and replacing the entire installation is likely your best course of action. Call a specialist at this point to be sure.
Removing Single Tiles
If you're dealing with an individual tile, remove it and replace it to get your tile floor up to speed. If it's only a tile or two you can do it on your own with basic hand-tools that are available for sale or rent at home improvement stores. The grout joints are best cut out with a handheld rotary tool and a burr bit, but you can also use an oscillating tool or an angle grinder in a pinch, or the hand-held grout removers where you scrape the grout out bit by bit. Cut out all the joints around the offending tile, pop it up out of the area with a hammer and chisel, scrape the thinset mortar away from the floor, replace it with fresh mortar and re-set the tile. Give it 24 hours to dry, grout it, and you should be good to go. Always wear safety goggles, a dust mask and gloves when removing tiles.
Sections are more labor-intensive to take care of, and may be worth calling in a professional if more than one section is present. Remove the grout joints around the perimeter of the area where you need to remove tiles. This keeps the vibrations from the tiles you are removing from affecting the good tiles surrounding the area. From there, pop the tiles up and remove the thinset mortar from the floor. With large areas, speed up your thinset removal with the use of an oscillating tool utilizing a scraper-type blade that easily vibrates and peels up sections of old thinset. From there, reinstall the tiles and get the floor back in use.
Fresh grout never dries the exact same color as grout from another mixture. This is due to the pigments within the grout being mixed at different ratios from batch to batch, and is why professionals always mix a full bag at a time even if it is too much rather than try and measure out portions for a small area. The color differences are subtle, but still noticeable. If your replacement tiles are in a highly visible area, it might be worth considering to remove the existing grout and re-grout the entire floor with a new batch to keep consistency.